Spark plugs generally have a center electrode connected with an ignition system as well as a side electrode spaced from the center electrode to establish a gap. The center electrode is surrounded by a ceramic insulator. When voltage generated by the ignition coil causes electrons to flow to the center electrode, a voltage difference develops between the center and the side electrode. The air and gasses in the gap act as an insulator, preventing voltage flow from the center electrode to the side electrode until the voltage exceeds the dielectric strength of the gasses in the gap, allowing electrons to flow across the gap, causing the gasses to react with each other to burn. The voltage at which electrons flow across the gap is referred to as the demand voltage.
If an insulating object, such as water, oil or debris, is in the gap or covers the electrode or counter electrode at the gap, the demand voltage increases. The demand voltage may go up so high as to exceed the dielectric strength of the ceramic insulator, causing it to chip. Ceramic chips may scratch the cylinder bore and allow oil to escape past the piston rings, causing excessive oil consumption.